A hydrodynamic rotary bearing comprises e.g. an outer sleeve, an inner ring and a lubricant. The outer sleeve and the inner ring are pivotable relatively to each other. Under normal operating conditions the inner ring and the outer sleeve of the hydrodynamic rotary bearing are separated by a film of the lubricant. At start-up and shut-down of the hydrodynamic rotary bearing, i.e. when the speed of rotation e.g. of the inner ring relatively to the outer sleeve is low, this film of the lubricant is in general not able to carry the load on the hydrodynamic rotary bearing, and the inner ring and the outer sleeve come into direct contact. This leads to increased wear and is therefore undesirable.
In FIG. 1 a sectional view of a hydrodynamic rotary bearing 1 is shown wherein the hydrodynamic rotary bearing 1 is at rest. The hydrodynamic rotary bearing comprises an outer sleeve 2 which may be provided with a bushing, an inner ring 3 e.g. designed to receive a shaft or journal and a lubricant 4. If the hydrodynamic rotary bearing 1 is mounted with its centre axis of rotation oriented substantially in the horizontal direction, the lubricant 4 tends to collect in the lower part of the hydrodynamic rotary bearing 1 when the hydrodynamic rotary bearing is at rest. In this constellation the inner ring 3 is pressed against the upper part of the outer sleeve 2. The direction and the point of attack of the loading force are indicated by the arrow F in FIG. 1. This situation is typical for a static, gravitational load supported by a rotating, horizontal shaft, as opposed to the “normal” situation where a rotating shaft is supported by a static structure.
In the situation illustrated in FIG. 1 the inner ring 3 and the outer sleeve 2 remain in direct and dry contact in the contact zone 5 with each other on start-up until the inner ring 3 has rotated enough to carry lubricant, e.g. oil, into the contact zone 5. Because the contact zone 5 is at the upper part of the hydrodynamic rotary bearing 1, the inner ring 3 may have to rotate by as much as half a revolution before a load carrying film of lubricant can begin to develop. Thus there occur wear in the contact zone 5 which consequently shortens the lifetime of the hydrodynamic rotary bearing 1.
In U.S. Pat. No. 6,781,276 B1 a wind turbine having a direct drive generator is disclosed. The generator comprises a stator arrangement and a rotor arrangement located inside the stator arrangement. The rotor arrangement is attached to a rotor shaft. The stator arrangement is supported on the rotor shaft by two bearings, which can be hydrodynamic rotary bearings. In this case the load of the stator arrangement is carried by the parts of the hydrodynamic rotary bearings which are above the rotor shaft. When a wind turbine, in particular a generator of this or a similar design is at rest, the lubricant of the hydrodynamic rotary bearings tends to drain to the bottom of the hydrodynamic rotary bearings under the influence of gravity. When the wind turbine, in particular the generator starts up, the entire load of the stator arrangement will initially be carried by dry bearing surface to bearing surface contacts in the two hydrodynamic rotary bearings. Because the rotor shaft or the rotor arrangement may have to rotate a significant distance before lubrication can take effect, the hydrodynamic rotary bearings are prone to serious wear.